Oxidative stress resulting from increased production of reactive oxygen species have been established as one of the mechanisms of the acute toxicity of TCDD and other halogenated polycyclic hydrocarbons. However, no experiments have investigated the role of oxidative stress in the chronic toxicity of these compounds. To assess the involvement of oxidative stress in the chronic toxicity of TCDD, effects of 26 weeks oral treatment with daily doses of 0.05 LD50 TCDD/kg and 0.005 LD50 TCDD/kg on the weight and histopathology of the liver, thymus, spleen, pancreas and adrenal from female C57BL/6J mice will be determined following staining with hematoxylin and eosin by two staff members in the Dept. of Pathology, Creighton University. Measures of oxidative stress including lipid peroxidation in the liver and extrahepatic tissue such as heart, testes, thymus, brain adrenal glands, and kidneys (measured by the thiobarbituric acid assay); DNA-single strand breaks in liver nuclei and peritoneal lavage cells (measured by DNA alkaline elution technique); production of reactive oxygen species by peritoneal lavage cells, liver mitochondria and liver microsomes (measured by the cytochrome c and iodonitrotetrazolium assays); production of nitric oxide (measured by nitrite concentration assay); activity of glutathione peroxidase; and effects on calcium and iron homeostasis will be determined following the above treatment with TCDD. Measures of TCDD induced immunotoxicity including secretion of interleukin-1 (IL-1) and secretion of tumor necrosis factor-alpha (TNF-a) by peritoneal lavage cells (measured by ELISA); secretion of interleukin-2 (IL-2) (measured by ELISA) and mixed leukocyte response by spleen single cell suspensions (SCS); and T-lymphocyte-mediated cytolysis (measured by 51CCr release assay) will also be determined following treatment of female C57BL/6J mice with oral daily doses of 0.05 LD50 TCDD/kg and 0.005 LD50 TCDD/kg for 26 weeks. The data from this study will provide a clear indication of the role of oxidative stress in the chronic toxicity of TCDD including its most sensitive target, the immune system.